\section{Conclusions}
\label{sec:conclusion}

In this study global simulations are performed with the ECHAM5 model 
to simulate radon activity 
in the lower troposphere and its effect on ion production. 
The decay chain of radon in the model is simplified 
%according to the half-life of the radon daughters. 
by removing short-lived radon daughters. 
The solution of the decay equation is computed analytically 
within each model time step, and coupled with tracer transport 
caused by advection, cumulus convection and turbulent mixing.
The radon-related ionization rate is estimated based on 
the activity concentration of radon and its daughter species, and  
well-accepted values of the decay/ionization energy.
 
Based on recent reports in the literature on radon emission, 
an up-to-date global radon emission map is
compiled with regional details and seasonal variation.   
The simulated radon activity concentration is 
evaluated against surface radon measurements at 51 locations. 
Results show that the global model ECHAM5 can reasonably reproduce 
the variations of surface radon concentrations observed at various locations.  
On the whole, the newly compiled emission map leads to better results 
compared to the WCRP1995 protocol and the widely used 
SW1998 map. The merged map is not only helpful for this study, 
but probably also useful for other researchers working on 
numerical modelling of radon transport and the transport and deposition 
processes of $^{210}$Pb \citep[e.g.,][]{balkanski:1993}.

The radon-related ionization rate is computed and compared with the 
GCR-ionization rate.  
It is found that in boreal winter, the suppressed vertical transport due to
increased atmospheric stability leads to seasonal mean IPRR as high as  
9~cm$^{-3}$~s$^{-1}$.   
In middle- and low-latitude continental areas, the zonal mean 
radon-induced ionization rate clearly exceeds the GCR-induced counterpart
in the near-surface levels up to 800~m elevation. 
At many continental sites, the observed and simulated surface 
radon activity concentration often occurs well above the 90th 
percentile of the equivalent concentration derived from the 
GCR-induced ionization. 
%
Further analysis on the joint PDF of ionization rate and temperature 
show that in China and USA, strong radon-related ionization 
often occur in winter at low ambient temperature, which provide 
favorable condition for the charged H$_2$SO$_4$/H$_2$O nucleation. 
In Russia, the ionization rate is not as high, but 
the very low and persistent winter temperature may play a more important 
role and still favor strong nucleation. Based on these results 
we conclude that it will be useful to extend the work of 
\citet{kazil:2010} to investigate the effect of radon-related 
ionization on nucleation, as well as the consequences in 
aerosol size distribution, cloud properties, and climate effect.



 




